Apparatus and process for making a corrugation-free foam

ABSTRACT

A method for extruding a corrugation-free polystyrene foam with up to 100% carbon dioxide as the blowing agent by extruding the extrudate through an annular die opening so that it contacts a choke ring inner surface. A foam made with the apparatus according to the invention will have no visible corrugations and an average cell size of less than 0.35 mm.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/418,165, filed Oct. 13, 1999, which claims priority on UnitedStates provisional patent application number 60/105,932, filed Oct. 28,1998.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to polymeric foams. In one aspect,the invention is directed to a process for producing a corrugation-freefoam using an annular die by extruding a mixture of a polymeric resinand a blowing agent through the die within a critical time. In anotheraspect, the invention relates to a process for making a fine cell sizefoam with a carbon dioxide blowing agent. The invention also relates toa process for making foam by extruding a polymeric resin through a chokering.

[0004] 2. Background of the Invention

[0005] Low density polymeric foam, such as polystyrene foam, isconventionally made by combining a physical blowing agent with a moltenpolymeric mixture under pressure and, after thorough mixing, extrudingthe combination through an appropriate die into a lower pressureatmosphere. This type of foam is commonly used to manufacture plates,bowls, cups and like items.

[0006] From about the 1950's to the present, physical blowing agents ofchoice have included halocarbons, hydrocarbons, specific atmosphericgases, or combinations thereof. Examples of the halocarbons includecommercially available halocarbon compositions such asdichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11) andmixtures thereof. Examples of the hydrocarbon blowing agents are theC₂-C₆ alkanes such as ethane, propane, butane, isobutane, pentane,isopentane, and hexane. Examples of the specific atmospheric gases arecarbon dioxide and argon.

[0007] During the 1980's, the worldwide scientific community presentedevidence linking halocarbons containing halogens other than fluorine,such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) withatmospheric ozone depletion. Consequently, governments sought toregulate CFCs and HCFCs. As a result of such regulations, manufacturersof extruded polymeric foam products were forced to switch toalternatives which have had adverse effects that resulted in increasedprocessing costs, reduced product quality, and increased safety issues,or combinations thereof.

[0008] For example, hydrocarbon blowing agents, particularly theshort-chained alkanes produce foam with satisfactory physicalproperties. However, depending upon the location of the factory and theamount of the blowing agent used, a manufacturer may be required tocapture and destroy emissions of the hydrocarbon blowing agents througha processing step like incineration. Atmospheric emission ofshort-chained hydrocarbons, which are classified as photoreactivevolatile organic compounds (VOCs), when combined with certain othergases and subjected to sunlight, may result in “smog”. Moreover, theflammability of the hydrocarbons requires elaborate control systems andcostly ventilation systems to prevent the exposure of highly flammableblowing agent-and-air mixtures to ignition sources. Similar tohydrocarbon blowing agents, certain hydrofluorocarbon blowing agents,such as 1,1-difluoroethane (HFC-152a), produce foam with satisfactoryphysical properties, but have the adverse effect of flammability. Inaddition, the nearly ten-fold higher unit pricing of thesehydrofluorocarbon blowing agents in relation to most of the hydrocarbonblowing agents adversely increases foam product costs.

[0009] The disadvantages of the prior blowing agents have led to the useof carbon dioxide as a blowing agent. Carbon dioxide does not have theadverse environmental and flammability characteristics associated withCFCs and HCFCs. Carbon dioxide has a molecular weight that is lower thanmost of the commercially used hydrocarbons and the hydrofluorocarbonsand thereby requires lower usage rates. Carbon dioxide also has lowerunit pricing than the commercially used hydrocarbons andhydrofluorocarbons. However, the foams made with higher levels of carbondioxide have not been comparable to foams made with hydrocarbon or withhydrofluorocarbon blowing agents. The foams made with blowing agentsthat are primarily carbon dioxide have generally had both increasedproduct cost and decreased product quality. The increased cost isattributable to a combination of reduced extrusion rates and limitedpost-expansion in secondary operations, which results in increasedproduct weight. The reduced product quality is attributable to bothdiminished aesthetics and increased variability in the mechanicalproperties.

[0010] The diminished aesthetics of foam produced with carbon dioxide isgenerally related to larger cell size, often greater than 0.4 mm, whichgive such foams a poor, grainy texture, and to the presence of multiplevisible parallel regions of light and dark in the foam substrate. Theseadjacent parallel regions are not only deleterious to the visibleaesthetics of the foams, but also create significant localizeddifferences to mechanical properties.

[0011] The physical property that both diminishes aesthetics andincreases the variability of the mechanical properties of foams madewith carbon dioxide is related to the low solubility of the carbondioxide gas in the polymer at ambient atmospheric conditions. The lowsolubility results in a very high volumetric expansion rate of thefoamable composition at the die. As a consequence of this highvolumetric expansion rate of the foam at the die, the use of a physicalblowing agent comprised primarily of carbon dioxide in the production offine-celled foams having a foam density below about 100 kg/m³ or a cellsize below about 0.40 mm causes corrugation. The severity of thecorrugations tends to increase as either the density or the cell size isdecreased. The corrugations are manifest as periodic bands which areoriented in the machine direction within the extruded foam sheet andwhich differ in cell size, cell shape, and often foam thickness from themajority of the foam. The corrugations not only detract from theaesthetics but also reduce the overall mechanical properties of partsmade from the foam.

[0012] In most food service and beverage applications, it is preferredthat the average cell size be about 0.20-0.30 mm, which provides thefoam with an aesthetically pleasing, relatively smooth surface texturewhile maintaining the requisite mechanical properties strength. Smallercell sizes tend to undesirably sacrifice a smoother finish for strength.Larger cell sizes tend to have an undesirable appearance.

[0013] When used as the sole blowing agent, carbon dioxide's very highvolumetric expansion rate typically produces unacceptable levels ofcorrugation. Therefore, previous attempts to use carbon dioxide as ablowing agent to produce a commercially acceptable foam product focusedon blending the carbon dioxide with another blowing agent. The blendedblowing agents typically included carbon dioxide as a minor constituentand either a hydrocarbon or hydrofluorocarbon blowing agent as thepredominant constituent. A common blended blowing agent would includecarbon dioxide in combination with pentane. Typically, the carbondioxide in the blended blowing agent was limited to 30 mole percent ofthe blended blowing agent, which reduced, but did not eliminate, the useof a hydrocarbon or hydroflurocarbon-blowing agent. Thus, the blendedblowing agent still has the disadvantages of the hydrocarbon andhydroflurocarbon blowing agents.

[0014] Attempts were also made to produce a commercially suitablepolystyrene foam with substantially 100 percent carbon dioxide as theblowing agent. Examples of such processes and foams are disclosed inU.S. Pat. Nos. 5,266,605, 5,340,844, and 5,250,577. Most of these foamshad an average cell size of 0.36 mm and still contained visiblecorrugation. Although these foams would be suitable for someapplications, they did not produce corrugation-free foams with cellsizes in the preferred range.

[0015] Referring to FIGS. 1 and 2, Applicants previously produced acorrugation-free polystyrene foam with 100 percent carbon dioxide as theblowing agent from a tandem extruder, which is commonly known in theindustry, in combination with a choke ring annularly positioned aroundan annular die extrusion opening.

[0016] The two-stage extruder apparatus comprises a hopper A feedingmaterial into a first extruder B where polystyrene resin material isheated and melted in heating zone C, mixed with a blowing agentdelivered by an injector D, further mixed by a mixing zone E, and cooledin a second stage extruder in a cooling zone F before delivery to a dieG. The choke ring H contacts the exiting extrudate before a sizingmandrel I sizes the sheet.

[0017] The previously-used choke ring 10 and die 12 are shown in moredetail in FIG. 2. The choke ring 10 has a smooth temperature-regulatedinner surface 11 that is positioned to be concentric with the die 12 sothat extrudate 13 contacts inner surface 11 before reaching the sizingmandrel 15.

[0018] The previously-used die 12 comprises a first generally convergingportion 16 that terminates at an annular die opening 18. A secondconverging portion 20 extends from the annular die opening 18 andterminates at a cylindrical portion 22.

[0019] The choke ring surface 11 and the annular die opening 18 arelocated a radius of r_(c) and r_(d), respectively, from the longitudinalaxis 24. The choke ring gap is the difference between the radii(r_(c)−r_(d)).

[0020] Two choke rings having different diameters were tried. The firstchoke ring had a diameter such that the gap was 0.2375 inches or 6.03millimeters. The second choke ring had a gap of 0.18 inches or 4.57millimeters, resulting in a contact time of approximately 0.37 ms forthe given operational parameters. Although both of these choke ringsproduced corrugation-free foam, the cell size remained above 0.40 mm.Therefore, there is still a need for a polystyrene foam and method ofmaking a polystyrene foam that is corrugation-free with a cell size inthe preferred range and using a carbon dioxide blowing agent.

SUMMARY OF INVENTION

[0021] The invention relates to a ethod for making ta orrugation-freefoam. The foam is preferably made in an extrusion apparatus thatextrudes a polymeric foam from an extrudate comprising a polymeric resinand a blowing agent. The blowing agent provides the extrudate with acellular structure upon extrusion. The apparatus can comprise anextruder having an inlet that is adapted to receive the extrudate. Anextrusion die is provided on the extruder and forms an outlet for theextruder. The extrusion die defines a longitudinal axis and has anannular die opening that is concentrically oriented relative to thelongitudinal axis and positioned a first radial distance from the axis.The apparatus further comprises a choke ring having an opening definedby an annular choke ring surface. The choke ring is positioned relativeto the extruder such that at least a portion of the die is receivedwithin the choke ring opening. The choke ring opening is preferablypositioned concentrically about the longitudinal axis and is positioneda second radial distance therefrom. The difference between the secondradial distance and the first radial distance defines the gap betweenthe choke ring and the die. The foam is extruded from the extrusion diethrough the choke ring at a preselected line speed. As the foam leavesthe extrusion die, it contacts the choke ring during an interval of timetermed the contact time.

[0022] The contact time is a function of the line speed and can becharacterized by the ratio of the gap size in millimeters (mm) relativeto the line speed in millimeters per second of the extrudate as itleaves the die opening with the ratio being between 0.001 and 0.020seconds. The choke ring gap is less than 4.6 mm and is preferably lessthan or equal to 0.8 mm. The gap can be specified as the differencebetween the first and second radial distances. It can be specified interms of the combination of a selected line speed and a ratio valuebetween 0.001 and 0.020 seconds. The gap size can also be specified interms of a selected contact time for a given range of line speeds.

[0023] The die can be of any suitable shape; however, it is preferredthat the die is symmetrical relative to the longitudinal axis. It isalso preferred that the die has a generally cylindrical body with anannular slot defining the die opening. The generally cylindrical bodycan include an annular ridge extending from the cylindrical body andterminating in a peak in which the annular slot is located.

[0024] In the method according to the invention, a polymeric foam isextruded from an extrudate that comprises a polymeric resin and ablowing agent, which provides the polymeric resin with a cellularstructure upon exiting from the extruder. The extruder has an annulardie with an annular die opening and a choke ring having an internalopening formed by an annular choke ring surface. The internal opening issized to receive at least a portion of the annular die. The annular diedefines a longitudinal axis from which the annular die opening is spaceda first radial distance and the annular choke ring surface is spaced asecond radial distance, with the difference between the second and firstradial distances defining a choke ring gap. The method includes formingthe extrudate by mixing a polymeric resin and a blowing agent and thenextruding the extrudate through the annular die opening at a rate sothat the extrudate leaving the annular die opening contacts the annularchoke ring surface within a contact time of 1.0-20.0 milliseconds (ms).

[0025] The extrudate is preferably kept constrained against the chokering surface for a constrainment time of 5-7 ms and preferably between8-50 ms. The blowing agent can be a blend but is preferably 100% carbondioxide. The rate, i.e. the line speed, at which the extrudate is pulledthrough the annular choke ring opening can range from 50 mm/second to250 mm/second. It is preferred that the contact time be less than 8 msand more preferably be 5 ms or less.

[0026] In an alternate form of the method, the polymeric foam is formedby mixing a polymeric resin and a blowing agent consisting substantiallyof 100% carbon dioxide, extruding the extrudate from the annular dieopening and through the choke ring gap, and controlling the extrudedfoam so it has an average cell size of 0.20 mm to 0.25 mm.

[0027] In another alternate form, the method relates to forming theextrudate by mixing a polymeric resin and a blowing agent comprising ablowing agent blend and extruding the extrudate through the choke ringgap.

[0028] The invention also relates to a foam, preferably made inaccordance with the above methods, having an average cell size of 0.20mm-0.35 mm. The average cell size is preferably less than 0.30 mm. Sucha foam will have no visible corrugation. Preferably, the foam will becorrugation-free. The foam can be made in a foam sheet, which has athickness preferably of 1-4 mm.

BRIEF DESCRIPTION OF DRAWINGS

[0029]FIG. 1 is a block diagram illustrating the major functionalcomponents of a prior art foam extrusion system, including a choke ringsurrounding an extrusion die;

[0030]FIG. 2 is an enlarged sectional view of the prior art choke ringand die of FIG. 1, illustrating the relationship between the choke ring,die, and sizing mandrel of the apparatus and the correspondinginteraction of the foam;

[0031]FIG. 3 is a view similar to FIG. 2 and illustrating a choke ringand die according to the invention; and

[0032]FIG. 4 is an enlarged sectional view showing the relationshipbetween the choke ring and die according to the invention.

DETAILED DESCRIPTION

[0033] The invention is both a corrugation-free foam and an apparatusfor making the corrugation-free foam. The corrugation-free foam ispreferably made using the prior art tandem extruder with an improvedchoke ring alone or in combination with an improved die. The improvementin the choke ring permits the production of a polystyrene foam with 100%carbon dioxide blowing agent having a fine cell structure with theaverage cell size being between 0.20 mm and 0.35 mm. It heretofore wasthought impossible to produce such a corrugation-free foam with such afine cell size by using carbon dioxide as a blowing agent.

[0034]FIGS. 3 and 4 illustrate the improved choke ring 30 along with thenew annular die 32. The choke ring 30 and annular die 32 are disclosedin the context of the prior art tandem extruder configuration of FIG. 1.Therefore, similar parts in the figures will be identified by the samenumerals.

[0035] The choke ring 30 has an annular inner surface 31 which isconcentrically oriented with respect to a longitudinal axis 44 so thatthe inner surface 31 is located a radial distance r_(c) from thelongitudinal axis 44. Similarly, the annular die 32 is concentricallyoriented with respect to longitudinal axis 44 and the die opening 38 ispositioned a radial distance r_(d) from the longitudinal axis 44. Thedifference between the radial distances, r_(c)−r_(d), is preferably lessthan 4.57 mm.

[0036] Unlike the prior art dies, the die 32 according to the inventiondoes not taper from the base to the tip of the die. Instead, the annulardie 32 has an outer periphery with an annular die opening 38 located inthe outer periphery and forming an outlet for the extruder. The outerperiphery of the extrusion die 32 is located at the annular die opening38. The die 32 begins at its base with a generally constant crosssection portion 34, which transitions into an outwardly directedradially converging portion or collar 36, in whose apex the annular dieopening 38 is formed. The die then ends with a slightly tapered crosssection portion 40. The advantage of the die 32 over the prior art die12 is that the die outlet opening forms the largest outer diameter ofthe die 32; no other portion of the die 32 can interfere with theinsertion of the die into the choke ring opening defined by the chokering inner surface 31, especially for the very small gap sizes requiredby the invention.

[0037] The choke ring 30 preferably comprises a smooth,temperature-regulated surface concentric to the annular foam diepositioned in such a manner as to direct but not reduce the flow of thefoamable extrudate as it leaves the die. Preferably an adjustmentmechanism of the choke ring apparatus allows the smooth,temperature-regulated surface 31 to be reproducibly positioned along theextrusion direction axis and be held concentric with the exit of die 32.An example of a suitable adjustment mechanism is one that can adjust theup/down, side-to-side, and upstream/downstream position of the chokering relative to the annular die 32. Worm gears or similar mechanism canbe used to make the adjustments.

[0038] It is also contemplated that the inner surface of the choke ringof the present invention can be constructed from any material that issolid at the temperature of the foamable extrudate. It is furthercontemplated that the inner surface of the choke ring can be constructedof any sintered material. The preferred materials of construction forthe choke ring are thermoplastic polymeric materials with a glasstransition temperature or melting temperature above about 160 C.,thermoset polymeric materials, and metallic materials. Examples ofsuitable thermoplastic materials include polytetrafluoroethylene,polyacetal, polyamides, polyesters, and polyoxymethylene, andcrosslinked polyolefins. Examples of thermoset materials includephenolics and epoxy resins. Examples of metallic materials includealuminum, carbon steel, and stainless steel. It is contemplated thatmaterials of higher thermal conductivity are more effective as materialsof construction of the inner surface of the choke ring. The mostpreferred material of construction for the inner surface of the chokering is aluminum.

[0039] It is contemplated that the inner surface of the choke ring ofthe present invention can be configured with any suitably-shaped curvethat will allow the expanding foamable composition to remaincontinuously constrained by the temperature-regulated surface for a timeperiod of from about 5 to about 75 ms. The choke ring inner surface canpreferably be configured to have diametrically opposing boundaries ofthe inner surface of the choke ring which are parallel lines,non-parallel lines, convex curves or concave curves. For examples inwhich the diametrically opposing sides of the choke ring are parallellines, the boundaries of the inner surface describe a cylinder. Forexamples in which the diametrically opposing sides of the choke ring arenon-parallel, the boundaries of the inner surface describe afrustoconical section of a cone.

[0040] The apex of the cone formed by a projection of the inner surfaceboundaries may be upstream or downstream of the die face. The term“converging angle” is used herein to describe the angle formed if theapex of the cone formed by the projection of the inner surfaceboundaries is downstream of the extruder. The term “diverging angle” isused herein to describe the angle formed if the apex of the cone formedby the projection of the inner surface boundaries is downstream of theextruder. In one example in which the diametrically opposing sides ofthe choke ring form a convex curve, the boundaries of the inner surfacedescribe a section of a paraboloid. The vertex of said paraboloid formedby projection of the choke ring inner surface boundaries may be upstreamor downstream of the die face. In one example in which the diametricallyopposing sides of the choke ring form a concave curve, the boundaries ofthe inner surface describe an inner section of a torus.

[0041] Preferred configurations for the inner surface 31 of the chokering are frustoconical sections with a diverging angle, cylinders, andfrustoconical sections with a converging angle. The converging angle ordiverging angle is measured using the extrusion direction axis as thebase. The most preferred configurations for the choke ring inner surfaceare a frustoconical section of a cone having a converging angle lessthan about 20°, a cylinder, and a frustoconical section of a cone havinga diverging angle less than about 30°. The most highly preferredconfigurations for the choke ring surface are a frustoconical section ofa cone having a converging angle less than 10°, a cylinder, and afrustoconical section of a cone having a diverging angle less than about10°.

[0042] It is contemplated that a downstream end 46 of the inner surfaceof the choke ring can be any configuration that will allow the foam tobegin radial expansion without tearing of the foam surface. Preferredconfigurations for the downstream end of the inner surface of the chokering are a radius in the range of from 0.4 mm to about 7.0 mm. The mostpreferred configuration for the downstream end 46 of the inner surfaceof the choke ring is a radius in the range of from about 1.5 mm to about3.5 mm.

[0043] It is further contemplated that the convective fluid used fortemperature regulation of the choke ring can be any fluid that isconventionally used for cooling or heating applications as long as saidfluid is not subject to thermal decomposition at the extrudatetemperature and said fluid will not react with the material ofconstruction of the choke ring. Examples of convective fluids includewater, ethylene glycol-water mixture in any proportions, and commerciallow viscosity thermal oils commonly used for heat transfer application.The preferred convective fluid for the temperature regulation of thechoke ring is water.

[0044] The support for the choke ring inner surface can be any supportthat will enable positioning of the inner surface to be concentric withthe annular die. The support can be attached to the extruder frame, thedie body or the floor. The support is preferably attached to the body ofthe die.

[0045] Broadly, the process of the present invention combines an alkenylaromatic polymer, a nucleating agent, a physical blowing agentconsisting of at least 15 mole percent carbon dioxide and optionally oneor more auxiliary physical blowing agents, and optional colorants andadditives in the extruder B to form a foamable alkenyl aromatic polymercomposition or extrudate. The foamable extrudate is pressurized above aparticular threshold pressure specific to the composition and releasedto an area of lower pressure through an annular die G. Thetemperature-regulated annular choke ring H is positioned so that, withina contact time period of about 20 ms or less after the foamableextrudate 13 exits the die G, the outer surface of the extruded material13 is forced into contact with the smooth inner surface 31 of the chokering 30 in a manner that deflects the outer surface of extrudate 13 butdoes not restrict the flow through the die 32 or damage the surface ofthe foam. The contact with the choke ring surface 31 is maintained for aconstrainment time period in the range of from about 5 to about 75 ms.The foam extrudate 13 is then allowed to expand freely in the radialdirection and is drawn at constant line speed over a mandrel 15 whichhas a radius of about 1.5 to about 6.0 times the radius of the annularchoke ring 10 to form a substantially corrugation-free alkenyl aromaticpolymer foam.

[0046] The corrugations sought to be eliminated or reduced by theinvention are well known in the art and comprise multiple parallelregions formed on the extruded sheet that are oriented in the extrusiondirection and which are apparent to the unaided eye of an observerthrough either visible light reflection or visible light transmission.The least severe corrugations are those in which the multiple parallelregions are visible only by transmitted light. Moderate corrugations arethose which are visible by reflected light and may have slight localizedthickness variations that are perceptible to human touch. Severecorrugations are those which also result in significant thicknessdifference between widths that are less than about 4 percent of theoverall sheet width. Extremely severe corrugation describes thecondition when adjacent parallel segments actually join together acrossthe width direction to produce an overlap or even “S” foldedcross-section in the thickness direction.

[0047] Mechanical properties of solid materials, such as flexuralstiffness and tensile strength, are directly relatable to the massdistribution of the substrate. Thus, mechanical properties of foam arelikewise directly dependent on the amount of solid mass or the localizeddensity of the foam. Corrugated foam generally has localized variationsin density. Consequently, since lower density means lower strength,corrugations are thus deleterious to the overall mechanical propertiesof the foam.

[0048] Looking at the process in more detail and according to oneembodiment of the present invention, the process for producingsubstantially corrugation-free alkenyl aromatic polymer foam begins byfeeding pellets of an alkenyl aromatic polymer into the extruder hopperA. The polymer along with 0.02 to about 2.0 weight percent of pelletizedtalc nucleating agent and 0 to about 2-percent of optional additives,colorants, and fire retardants are fed by gravity into the hopper A.(All weight percentages relate to the weight of the extrudate, unlessotherwise noted.) The polymeric-talc-additives mixture is conveyedthrough the hopper A into the first extruder B and heated at the heatingzone C to a temperature sufficient to form a polymeric-talc-additivesblend.

[0049] A physical blowing agent consisting preferably solely of carbondioxide and optionally one or more members selected from the groupconsisting of fully hydrogenated hydrocarbon blowing agents, partiallyfluorinated blowing agents, and combinations thereof, is added at theinjector D of the extruder in an appropriate ratio to the targetdensity. Preferably, the carbon dioxide comprises 1 to 3 weight percentand is injected in a liquid state. The polymeric-talc-additives blendand physical blowing agent are thoroughly mixed in the mixing zone E,transferred to the second extruder, and subsequently cooled in a coolingzone F to a temperature sufficient to form foam. The cooled foamableextrudate consisting of the polymeric-talc-additives-physical blowingagent is extruded through an annular die into a lower pressure region onthe downstream side of the die G.

[0050] It is worth noting that carbon dioxide as used herein refers tocommercially available carbon dioxide. Commercially available carbondioxide is not pure and contains some contaminants. It is preferable touse 100% carbon dioxide.

[0051] As shown in FIGS. 3 and 4, the expansion of the extruded foamablecomposition is restricted by the choke ring 30, while making contactwith the smooth temperature-regulated inner surface 31, which isconcentric with the annular die 32. The inner surface 31 is sized sothat the foamable composition contacts the inner surface of the chokering within a contact time period of between about 1 and 20 ms. The foamis drawn over the inner surface 31 of the choke ring 30 and held thereagainst it for a constrainment time period of between about 5 and 75 ms.The resulting foam 13 is then allowed to expand freely in a radialdirection in the form of an air bubble 14 of regulated pressure, drawnover a cylindrical sizing mandrel 15 that has a diameter that is 1.5 to6.0 times that of the annular die, and collected on any conventionalsheet collection device. The resulting alkenyl aromatic polymer foam ofthe present invention is free of visible corrugations. The foampreferably has no corrugations visible to the eye by light transmission.

[0052] The polymeric foams produced by the present invention aregenerally of a density of from about 30 kg/m³ to 120 kg/m³. Thepolymeric foams produced by the present invention generally have anaverage cell size from about 0.20 mm to about 0.35 mm. The polymericfoams are produced with uniform and consistent physical properties. Thepolymeric foams are substantially free of even the least severecorrugations. The polymeric foams are light in weight and can beconverted into plates, cups, and bowls. Other contemplated applicationsfor the polymeric foams produced by the present invention include usesin insulation, toys, and low impact protective packaging applications.

[0053] The polymeric foam produced by the present invention preferablyhas a thin cross-section in the thickness direction of the foamedstructure that is less than about 5 mm. The preferred dimension in thethickness direction of the foamed product is from about 1.0 to 4.0 mm.

[0054] The “average cell size” as used herein is defined as the mean ofthe cross direction cell size and the extrusion direction cell size. Theextrusion direction cell size and cross direction cell size are measuredin conformance to ASTM Method D3676. The average cell size is in therange of about 0.15 to about 0.60 mm. The most preferred average cellsize is in the range of about 0.20 to about 0.35 mm in order to obtainthe most commercially desirable combination of finish, strength, anddensity.The contact time is a characteristic time period required by anelement of the foamable composition on the outer surface of theexpanding foamed structure to travel the distance from the annular dieoutlet opening to the inner surface 11 of the choke ring 10. The contacttime is calculated by dividing the distance between the choke ring innersurface and the die outlet (in units of length such as mm) by the linespeed (in units of length/time such as mm/sec) used in drawing the foamover the mandrel.

[0055] That is:

t _(c0)=(r _(c) −r _(d))/L

[0056] where

[0057] t_(c0) is the contact time (in sec),

[0058] r_(c) is the mean radius of the choke ring inner surface (mm),

[0059] r_(d) is the radius of the annular die at the die gap (mm), and

[0060] L is the mean line speed of downstream equipment (mm/sec)

[0061] The preferred contact time is from about 1.0 to about 20 ms. Themost preferred contact time is from about 1.0 to 8 ms. The most highlypreferred contact time is from 1.0 to 5.0 ms. A preferred range of linespeeds is 50 to 300 mm/s, with the most preferred line speed being150-250 mm/s.

[0062] The constrainment time is preferably a characteristic time periodthat an element of the foamable composition on the outer surface of thefoamed structure actually travels along the inner surface of the chokering. The constrainment time is calculated by dividing the length of thechoke ring inner surface that is downstream of the annular die exit (inunits of length such as mm) by the line speed (in units of length/timesuch as mm/sec) used in drawing the foam over the mandrel.

[0063] That is:

t _(c1) =l _(c) /L

[0064] where

[0065] t_(c1) is the constrainment time (in sec),

[0066] l_(c) is the contact length of the choke ring inner surface (mm),

[0067] L is the mean line speed of downstream equipment (mm/sec)

[0068] The preferred constrainment time is from about 5 ms to about 75ms. The most preferred constrainment time is from about 8 to about 50ms. A preferred range of line speeds is 50 to 300 mm/s, with the mostpreferred line speed being 150-250 mm/s.

[0069] The alkenyl aromatic polymer preferably includes polymers ofaromatic hydrocarbon molecules that contain an aryl group joined to anolefinic group with only double bonds in the linear structure, such asstyrene, α-methylstyrene, o-methylstyrene, m-methylstyrene,p-methylstyrene, α-ethylstyrene, α-vinylxylene, α-chlorostyrene,α-bromostyrene, vinyl toluene and the like. Alkenyl aromatics polymersinclude homopolymers of styrene (commonly referred to as polystyrene)and copolymers of styrene and butadiene (commonly referred to as impactpolystyrene).

[0070] The contact time, constrainment time, the choke ring gap size,and the line speed are interdependent or, in other words, functions ofeach other. Although the contact time and constrainment time are usefulfor quantifying operational range of the process for obtaining acorrugation-free foam according to the invention, testing has also shownthat the currently preferred choke ring gap of 0.03 inches producescorrugation-free foam with a 0.20-0.35 mm average cell size for thecurrent range of line speeds.

[0071] The polystyrene resin or polystyrenic material preferablyincludes homopolymers of styrene, and styrene copolymers comprised of atleast 50 mole percent of a styrene unit (preferably at least about 70mole percent) and a minor (i.e. less than 50 mole percent) proportion ofa monomer copolymerizable with styrene. The term “polystyrene resin” or“polystyrenic material” as used herein also includes blends of at least50 percent by weight of the styrene homopolymer (preferably at leastabout 60 weight percent) with another predominantly styrenic copolymer.The physical blends are combined in a dry form after the blends havebeen polymerized.

[0072] The polystyrene resin that can be used in the polymeric mixturecan be any of those homopolymers obtained by polymerizing styrene to aweight average molecular weight (M_(W)) of from about 100,000 to about450,000 (commonly referred to as crystal polystyrene), can be any ofthose copolymers obtained by polymerizing styrene and from about 3 to 20mole percent butadiene to a weight average molecular weight (M_(W)) offrom about 100,000 to about 350,000, or can be any of those graftcopolymers obtained by polymerizing a blend of polymerized styrene upona nucleus of styrene-butadiene rubber (SBR) to a weight averagemolecular weight of from about 100,000 to about 350,000 (commonlyreferred to as impact polystyrene).

[0073] The preferred crystal polystyrenes are uncrosslinked homopolymersof styrene and have a melt flow index of from about 0.5 to about 15.0dg/min. as measured by ASTM D1238 (nominal flow rate at 200 C. and 689.5kPa). The most preferred crystal polystyrene is uncrosslinkedpolystyrene having a melt flow index from about 1.0 to 3.0 dg/min.

[0074] Impact polystyrenes are generally classified as medium impactpolystyrene (MIPS), high impact polystyrene (HIPS) or super high impactpolystyrene (S-HIPS). The butadiene level of the impact polymer ispreferably in the range of from about 3 to about 10 weight percent ofthe copolymer (polybutadiene and polystyrene). The most preferredbutadiene level is in the range of from about 5 to 8 weight percent ofthe copolymer. The impact polystyrene generally has a melt flow index ofless than about 25 dg/min., and preferably less than about 8 dg/min. Themost preferred impact polystyrenes are uncrosslinked HIPSs having a meltflow index of from about 2.2 to 3.2 dg/min. as measured by ASTM D1238(nominal flow rate at 200 C. and 689.5 kPa), and a Notched Izod Impactof from about 9 to about 13 kg-cm/cm as measured by ASTM D256. TheNotched Izod Impact is the energy required to break notched specimensunder standard conditions and is work per unit of notch. Therefore, ahigher Notched Izod Impact indicates a tougher material.

[0075] The alkenyl aromatic polymer of the present invention can beobtained by blending two or more alkenyl aromatic polymers. For example,blends of crystal polystyrene and impact polystyrenes such as crystalpolystyrene and HIPS, may be blended to comprise the alkenyl aromaticpolymer of the present invention.

[0076] The nucleating agent preferably includes any conventional oruseful nucleating agent(s) used to adjust the size of the cells in thefoamed structure to the target size desired. The term “cell size controlagent” has also been used interchangeably in the art. The amount ofnucleating agent to be added depends upon the desired cell size, theselected blowing agent, and the density of the alkenyl aromatic polymerfoam composition. The nucleating agent is generally added in amounts offrom about 0.02 to 2.0 weight percent of the polymeric composition.Nucleating agents may be inorganic or organic compounds and aregenerally available in a small particulate form.

[0077] Examples of inorganic nucleating agents include clay, talc,silica, and diatomaceous earth. The preferred organic nucleating agentsinclude those compounds which decompose or react at the heatingtemperature within the extruder to evolve gas. Examples of thesepreferred organic nucleating agents include polycarboxylic acids andalkali metal salts of a polycarboxylic acid in combination with acarbonate or bicarbonate. Some specific examples of an alkali metal saltinclude, but are not limited, to the monosodium salt of2,3-dihydroxy-butanedioic acid (commonly referred to as sodium hydrogentartrate), the monopotassium salt of butanedioic acid (commonly referredto as potassium hydrogen succinate), the trisodium and tripotassiumsalts of 2-hydroxy-1,2,3-propanetriccarboxylic acid (commonly referredto as sodium and potassium citrate respectively), and the disodium saltof ethanedioic acid (commonly referred to as sodium oxalate). An exampleof a polycarboxylic acid is 2-hydroxy-1,2,3-propanetricarboxylic acid(commonly referred to as citric acid). Some examples of a carbonate or abicarbonate include, but are not limited to, sodium carbonate, sodiumbicarbonate, potassium carbonate, and calcium carbonate.

[0078] It is contemplated that mixtures of inorganic and organicnucleating agents can also be used in the present invention. The mostpreferred nucleating agent is talc. Talc is preferably added in a powderform, but may also be added in a carrier. If added in a carrier, thetalc concentration is preferably between 20 to 60 weight percent in analkenyl aromatic polymer which is preferably a styrene homopolymer.

[0079] The physical blowing agent for this invention includes at least15 mole percent, preferably at least 50 mole percent, carbon dioxide andoptionally one or more auxiliary physical blowing agents. The mostpreferred amount is 100 percent carbon dioxide to provide the greatestpositive environmental and safety characteristics. The carbon dioxideblowing agent can be used at a rate of about 0.1 to 4.0 weight percent,but preferably about 1.0 to about 3.0 weight percent, of the totalextruder feed rate.

[0080] The auxiliary physical blowing agent comprises at least 1 molepercent, and preferably at least 5 mole percent, but less than 85 molepercent of the total blowing agent. More than one auxiliary physicalblowing agents may also be included.

[0081] Examples of auxiliary physical blowing agents include but are notlimited to organic physical blowing agents.

[0082] The organic auxiliary physical blowing agents preferably includesorganic chemical compounds that have boiling points less than about 37C. These organic compounds include, but are not limited to, fullyhydrogenated hydrocarbons and partially fluorinated hydrocarbons whichmay be considered to be flammable. Flammable as defined herein generallyincludes those materials having flashpoints less than about 37.8 C.

[0083] Examples of fully hydrogenated hydrocarbon blowing agents includethe initial members of the alkane series of hydrocarbons that contain upto six carbon atoms. Preferably, the hydrogenated hydrocarbon blowingagents are not regulated by governmental agencies as being specificallytoxic to human or plant life under normal exposure. The preferred C₁-C₆alkane compounds include methane, ethane, propane, n-butane, isobutane,n-pentane, isopentane, n-hexane, and blends thereof. The most preferredfully hydrogenated hydrocarbon auxiliary physical blowing agents are theC₄-C₅ and blends thereof.

[0084] The preferred partially fluorinated hydrocarbons auxiliaryphysical blowing agents are hydrofluorocarbon gases that have moleculeswhich contain up to three carbon atoms without any other halogen atomsother than fluorine. These partially fluorinated auxiliary physicalblowing agents may be flammable. The most preferred partiallyfluorinated hydrocarbon auxiliary physical blowing agents are1,1-difluorethane (HFC-152a) and 1,1,1-trifluoroethane (HFC-143a). It isalso contemplated that 1,1-chloroethane (HFC-142b) and1-1-dichloro-2-fluoroethane (HFC-141b) may be added as auxiliary blowingagents for non-regulated insulation applications.

[0085] The optional additives used in the process preferably providespecific, non-mechanical physical properties to the foamed product anddo not interfere with or influence the extrusion of the foamed product.Additives may constitute from about 0.05 to about 5 weight percent ofthe total polymeric foam rate. Examples of additives include but are notlimited to antistats, fire retardants, perfumes, ultra-violet (UV) lightabsorber, UV stabilizers, and infra-red light tracers.

[0086] Similarly, colorants used in the process preferably includespecific additives included solely for the purpose of providing adesired color to the foamed product that is different from the naturalcolor provided by the alkenyl aromatic polymer when foamed. Examples ofcolorants include various pigments and color concentrates as known inthe art, such as carbon black and titanium dioxide white. The preferredform of colorant for addition to the extruder is in a pellet consistingof about 1 to about 40 weight percent of the color material compoundedin an alkenyl aromatic polymer such as polystyrene that may be differentfrom the alkenyl aromatic polymer used for the foam. The most preferredform of colorant for addition to the extruder is in a pellet consistingof about 5 to about 20 weight percent of the color material compoundedin the same alkenyl aromatic polymer as used for the foam. The mostpreferred concentration of additive is from 0.2 to 2.0 weight percent ofthe total extruder flow rate.

INVENTIVE EXAMPLE 1

[0087] Pellets of a previously extruded mixture of Dart Polymers, Inc.PS101 high heat crystal polystyrene (specific gravity of about 1.05g/cm³ and a melt index (MI) of about 1.8 dg/min.) and an undeterminedlevel, between 1.0 and 5.0 weight percent, of Phillips K-Resin(styrene-butadiene copolymer) was mixed with 0.50 weight percent ofHuntsman 27678 1 talc concentrate pellets. The pellet mixture was heatedto form a blend in a 32:1 L:D Battenfeld Gloucester Engineering Co.,Inc. 2.5-inch (35.3 cm) single-screw extruder operating at a screw speedof about 95 rpm. Pressurized commercial-grade carbon dioxide (31.0 MPa)was injected at a rate of 2.31 kg/hr. The polymer melt and carbondioxide were mixed and further heated to a melt temperature of about 209C. and pressurized to 26.9 MPa at the extruder discharge.

[0088] The heated mixture was then transferred through a heated pipe toa second, larger 3.5 inch (89 mm) single-screw cooling extruderoperating at 25 rpm. Subsequently, the extrudate was cooled to a melttemperature of about 149 C. and pressurized to about 21.4 MPa fordelivery at about 75 kg/hr into a 5.40-cm diameter annular die.

[0089] The extrudate is pulled from the die by downstream equipmentwhich is operating at about 244 mm/sec (9.61 inches/sec) and is drawninto contact with a choke ring having an inner surface diameter of 5.55cm (2.19 inches) and a choke ring gap size of 0.762 mm (0.03 inch), fora contact time of 3.13 ms. The choke ring temperature was regulated bythe flow of cooling water which was maintained at 27 C. The foamremained in contact with the inner surface of the choke ring for adistance of about 6.4 mm, resulting in a constrainment time of about 26ms. The foam was then allowed to expand freely and was subsequentlydrawn over a mandrel to form a foam having a density of 59.0 kg/m , anaverage thickness of 2.22 mm, and an average cell size of 0.25 mm. Thefoam is free of corrugations visible to the unaided eye by lighttransmission.

INVENTIVE EXAMPLE 2

[0090] This example is similar to Inventive Example 1 with reduction ofthe talc concentrate level to 0.25 weight percent and an increase of thecooling water temperature on the choke ring to 63 C. and a reduction ofthe line speed to 199 mm/sec.

[0091] The contact time was 3.83 ms. The constrainment time was about 32ms. The foam was then allowed to expand freely and was subsequentlydrawn over a mandrel to form a foam having a density of 60.0 kg/m , anaverage thickness of 2.57 mm, and an average cell size of 0.35 mm. Thefoam is free of corrugations visible to the unaided eye by lighttransmission.

INVENTIVE EXAMPLE 3

[0092] This example is similar to Inventive Example 2 with a change to ablowing agent blend comprising 80 mole percent carbon dioxide and 20mole percent Phillips Chemical Company commercial grade isopentane at atotal injection rate of 2.54 kg/hr. Other adjustments were a coolingextruder screw speed of 26 rpm, choke ring cooling water temperature to32 C. and line speed to 203 mm/sec.

[0093] The contact time was 3.75 ms. The constrainment time was about 31ms. The foam was then allowed to expand freely and was subsequentlydrawn over a mandrel to form a foam having a density of 60.6 kg/m³, anaverage thickness of 2.56 mm, and an average cell size of 0.31 mm. Thefoam is free of corrugations visible to the unaided eye by lighttransmission.

INVENTIVE EXAMPLE 4

[0094] This example is similar to Inventive Example 3 with a change to ablowing agent blend comprising 62 mole percent carbon dioxide and 38mole percent Phillips Chemical Company commercial grade isopentane at atotal physical blowing agent injection rate of 2.27 kg/hr. The linespeed was changed to 221 (8.70 inches/sec).

[0095] The contact time was 3.45 ms. The constrainment time was about 29ms. The foam was then allowed to expand freely and was subsequentlydrawn over a mandrel to form a foam having a density of 82.2 kg/m³, anaverage thickness of 1.78 mm, and an average cell size of 0.31 mm. Thefoam is free of corrugations visible to the unaided eye by lighttransmission.

INVENTIVE EXAMPLE 5

[0096] This example is similar to Inventive Example 1 with reduction ofthe carbon dioxide rate to 1.95 kg/hr, an increase of the choke ringcooling water temperature to 32 C. and a reduction of the line speed to156 mm/sec (6.14 inches/sec).

[0097] The contact time was 4.89 ms. The constrainment time was about 41ms. The foam was then allowed to expand freely and was subsequentlydrawn over a mandrel to form a foam having a density of 70.1 kg/m³, anaverage thickness of 2.87 mm, and an average cell size of 0.30 The foamis free of corrugations visible to the unaided eye by lighttransmission.

COMPARATIVE EXAMPLE 6

[0098] This example is similar to Inventive Example 2 with theelimination of the choke ring and change of the nucleating agent from atalc concentrate to a powdered talc. The extrusion rate was 80.7 kg/hr.

[0099] The foam was allowed to expand from the die freely and wassubsequently drawn over a mandrel to form a foam having a density of59.9 kg/m³, an average thickness of 3.25 mm, and an average cell size of0.59 mm. The foam has large cells and moderate corrugations that havevisible thickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 7

[0100] This example is similar to Comparative Example 6 with a change ofthe nucleating agent to Boehringer Ingelheim Hydrocerol Compound (amixture of citric acid and sodium bicarbonate in a proprietary carrier).

[0101] The foam was allowed to expand from the die freely and wassubsequently drawn over a mandrel to form a foam having a density of85.7 kg/m³, an average thickness of 1.49 mm, and an average cell size of0.34 mm. The foam has large cells and severe corrugations that havevisible thickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 8

[0102] This example is similar to Inventive Example 3 with theelimination of the choke ring. The foam was allowed to expand from thedie freely and was subsequently drawn over a mandrel to form a foamhaving a density of 74.7 kg/m³, an average thickness of 1.84 mm, and anaverage cell size of 0.35 mm. The foam has large cells and moderatecorrugations that have visible thickness variations on the surface ofthe foam.

COMPARATIVE EXAMPLE 9

[0103] This example is also similar to Comparative Example 8 with achange of carbon dioxide/isopentane mole fraction ratio from 80/20 to70/30. The foam was allowed to expand from the die freely and wassubsequently drawn over a mandrel to form a foam having a density of79.8 kg/m³, an average thickness of 1.63 mm, and an average cell size of0.29 mm. The foam has large cells and moderate corrugations that havevisible thickness variations on the surface of the foam.

COMPARATIVE EXAMPLE 10

[0104] This example is similar to Inventive Example b 1 except that thechoke ring had an inner surface diameter of 66.04 mm, resulting in a gapof 6 mm (0.24 inches) and a contact time of 38 ms. The resultant foamwas corrugation-free and had a thickness of 2.91 mm, a density of 66.4kg/m , and an average cell size of 0.42 mm.

COMPARATIVE EXAMPLE 11

[0105] This example is similar to Inventive Example 1 except that achoke ring having an inner surface diameter of 63.12 mm, resulting in agap of 4.6 mm (0.18 inches) and a contact time of 20 ms, was used. Theresultant foam has severe corrugation and had a thickness of 2.46 mm, adensity of 54.1 kg/m³, and an average cell size of 0.26 mm.

COMPARATIVE EXAMPLE 12

[0106] Comparative Example 12 is similar to Comparative Example 11except that the contact time was 30 ms, which resulted in a foam havingsevere corrugation with a thickness of 3.08 mm, a density of 65.8 kg/m²,and an average cell size of 0.27 mm.

[0107] The key results of the examples are summarized in Table 1. TABLE1 Physical Blowing Agent CO₂ Isopentane Extruded Average Example MoleMole Density Cell Size Thickness Number Fraction Fraction (kg/m³) (mm)(mm) Corrugation INVENTIVE FOAMS 1 100% 0% 59.0 0.25 2.22 None 2 100% 0%60.0 0.35 2.57 None 3  80% 20%  60.6 0.31 2.56 None 4  62% 38%  82.20.27 1.78 None 5 100% 0% 70.1 0.30 2.87 None COMPARATIVE FOAMS 6 100% 0%59.9 0.59 3.25 Moderate 7 100% 0% 85.7 0.34 1.49 Severe 8  80% 20%  74.70.35 1.84 Moderate 9  70% 30%  79.8 0.29 1.63 Moderate 10 100% 0% 66.40.42 2.91 None 11 100% 0% 54.1 0.26 2.46 Severe 12 100% 0% 65.8 0.273.08 Severe

[0108] As can be seen, the invention with its choke ring and reducedcontact times result in a highly desirable commercial foam having a cellsize within the preferred range of 0.20 to 0.35 without any corrugation,regardless of the percentage of carbon dioxide. The invention is furtheradvantageous in that the blowing agent can comprise 100 percent CO₂which has superior environmental and safety characteristics as comparedto the hydrocarbon and hydroflurocarbon blowing agents. The radialdistance r_(c) for a choke ring providing an acceptable foam is lessthan 30 mm, preferably less than 28 mm. The line speed is less than 300mm/second, preferably between about 150-250mm/s. The prior art foamsmade with a choke ring having contact times greater than 20 ms did notproduce a suitable foam because either the corrugation or cell size wastoo great. The prior art foams made without a choke ring were notsuitable because both the corrugation and the cell size were too great.

[0109] While the present invention has been described with reference toone or more particular embodiments, those skilled in the art willrecognize that many changes may be made thereto without departing fromthe scope and intentions of the present invention. Those variationsthereof are contemplated to fall within the scope and intention of thedescribed invention.

1. A method of extruding a polymeric foam from an extrudate comprising apolymeric resin and a blowing agent for providing the polymeric resinwith a cellular structure upon exiting from an extruder having anannular die with an annular die opening and a choke ring having a chokering aperture formed by an annular choke ring surface and sized toreceive at least a portion of the annular die, the annular die defininga longitudinal axis from which the annular die opening is spaced a firstradial distance and the annular choke ring surface is spaced a secondradial distance, the method comprising: forming the extrudate by mixinga polymeric resin and a blowing agent; and extruding the extrudatethrough the annular die opening so that the extrudate leaving theannular die opening contacts the annular choke ring surface within acontact time of 1.0 to 20.0 milliseconds (ms).
 2. The method of claim 1and further comprising the step of constraining the extruded foam incontact with the choke ring for a constrainment time of 5 to 75 ms. 3.The method of claim 2 wherein the constrainment time is 8 to 50milliseconds.
 4. The method of claim 1 wherein the blowing agent issubstantially 100% carbon dioxide.
 5. The method of claim 2 wherein theblowing agent is a blend comprising carbon dioxide.
 6. The method ofclaim 5 wherein the blend comprises carbon dioxide and pentane.
 7. Themethod of claim 6 wherein the blend comprises less than 50% pentane. 8.The method of claim 1 wherein the extrudate is pulled away from the dieat a line speed of 50 mm/s to 250 mm/s.
 9. The method of claim 1 whereinthe polymeric resin is a styrenic resin.
 10. The method of claim 9wherein the styrenic resin is polystyrene.
 11. The method of claim 1wherein the contact time is less than 8.0 ms.
 12. The method of claim 11wherein the blowing agent is substantially 100% carbon dioxide.
 13. Themethod of claim 12 wherein the step of forming the extrudate includesmixing a nucleating agent into the extrudate prior to the extrudingstep.
 14. The method of claim 1 wherein the resultant foam has novisible corrugation.
 15. A method of extruding a polymeric foam from anextrudate comprising a polymeric resin and a blowing agent for providingthe polymeric resin with a cellular structure upon exiting from anextruder having an annular die with an annular die opening and a chokering having a choke ring opening formed by an annular choke ring surfaceand sized to receive at least a portion of the annular die, theextrusion die defining a longitudinal axis from which the annular dieopening is spaced a first radial distance and the annular choke ringsurface is spaced a second radial distance wherein the gap between thechoke ring surface and the die is equal to the difference between thesecond radial distance and the first radial distance, the methodcomprising: forming the extrudate by mixing a polymeric resin and ablowing agent consisting substantially of 100% carbon dioxide; andextruding the extrudate through the choke ring gap so that the resultantextruded foam has an average cell size of 0.20 to 0.35 millimeter (mm).16. The method of claim 15 wherein the extruding step yields a foamhaving an average cell size of less than 0.30 mm.
 17. The method ofclaim 16 wherein the polymeric resin is polystyrene.
 18. The method ofclaim 15 wherein the extruding step includes pulling the extrudatethrough the die opening at a rate so that the extrudate contacts thechoke ring surface within a contact time of 1.0 to 20.0 milliseconds(ms) after leaving the annular die opening.
 19. The method of claim 18wherein the contact time is 1.0 to 8.0 ms.
 20. The method of claim 15wherein the extruding step yields a foam having an average cell size ofless than 0.30 mm.
 21. The method of claim 20 wherein the foam has novisible corrugation.
 22. The method of claim 21 wherein the foam issubstantially corrugation-free.
 23. The method of claim 15 wherein thefoam is in sheet form.
 24. The method of claim 23 wherein the foam sheethas a thickness between 1.0 to 4.0 mm.
 25. The method of claim 24wherein the foam sheet has a density of 30 to 120 kg/m³.
 26. The methodof claim 15 and further comprising the step of extruding the extrudatethrough the annular die opening so that the extrudate leaving theannular die opening contacts the annular choke ring surface within acontact time of 1.0 to 20.0 milliseconds.
 27. The method of claim 26wherein the foam has no visible corrugation.
 28. The method of claim 26wherein the contact time is 1.0 to 8.0 milliseconds.
 29. The method ofclaim 15 wherein the foam has no visible corrugation.
 30. The method ofclaim 15 and further comprising the step of constraining the extrudedfoam in contact with the choke ring for a constrainment time of 5 to 75milliseconds.
 31. The method of claim 15 and further comprising the stepof blending pentane with the carbon dioxide to form the blowing agent.32. The method of claim 15 and further comprising the step of pullingthe extrudate away from the die at a line speed of 50 millimeters persecond to 250 millimeters per second.
 33. The method of claim 15 andfurther comprising the step of mixing a nucleating agent into theextrudate prior to the extruding step.
 34. A method of extruding apolymeric foam from an extrudate comprising a polymeric resin and ablowing agent for providing the polymeric resin with a cellularstructure upon exiting from an extruder having an annular die with anannular die opening and a choke ring having a die aperture openingformed by an annular choke ring surface and sized to receive at least aportion of the annular die, the extrusion die defining a longitudinalaxis from which the annular die opening is spaced a first radialdistance and the annular choke ring surface is spaced a second radialdistance wherein the gap between the choke ring and the die is equal tothe difference between the second radial distance and the first radialdistance, the method comprising: forming the extrudate by mixing apolymeric resin and a blowing agent comprising a blowing agent blend;and extruding the extrudate from the die opening and through the chokering gap.
 35. The method according to claim 34 and further comprisingthe step of selecting a choke ring gap of less than 4.6 mm.
 36. Themethod according to claim 35 wherein the step of selecting the chokering gap comprises selecting the choke ring gap less than 0.8 mm. 37.The method according to claim 34 wherein the extruding step furthercomprises pulling the extrudate from the die opening at a line speed sothat the extrudate leaving the annular die opening contacts the annularchoke ring surface within a contact time of 1.0 to 20.0 milliseconds.38. The method of claim 37 and further comprising the step ofconstraining the extruded foam in contact with the choke ring for aconstrainment time of 5 to 75 milliseconds.
 39. The method of claim 38wherein the constrainment time is 8 to 50 milliseconds.
 40. The methodof claim 34 wherein the extruding step includes extruding the extrudatethrough the choke ring gap such that the resultant foam has no visiblecorrugations.